A. Polyesters in General
Polyesters are the polycondensation products of a dicarboxylic acid and a dihydroxy alcohol. For example, the most widely used polyester in the world, polyethylene terephthalate, is formed by reacting terephthalic acid (or dimethyl terephthalate) with ethylene glycol: ##STR1## Saturated, linear polyesters, such as polyethylene terephthalate, are flexible and are used primarily to make fibers. Polyester fibers are, in turn, used as clothing fabric (Dacron is a trademark of E. I. duPont de Neumours & Co. for a polyester fiber made from polyethylene terephthalate), tire fabric, seat belts etc.
To impart thermosetting properties to polyesters, it is often desirable to "cross link" the linear polymer chains. A cross-linked polyester is generally prepared by first introducing ethylenic unsaturation into the chains by the use of an unsaturated dicarboxylic acid, e.g., maleic acid. Next, the unsaturated polyester chain is dissolved in a vinyl monomer, e.g., styrene. The addition of a vinyl polymerization catalyst then catalyzes the copolymerization and the chains are cross-linked. This two-step process can be represented as follows: ##STR2##
The unsaturated polyester resins are often handled as solutions in a vinyl monomer. They have the ability, when catalyzed, to copolymerize (cross-link) under atmospheric pressure and at moderate temperatures to form rigid plastics. The hardening resulting from the cross-linking is often referred to as "curing". These resins are widely used to fabricate plastic items, often in combination with fiber glass or other reinforcing materials, and are also used as coatings and laminates.
The unsaturated polyester resins are relatively expensive and are generally derived from petroleum feedstocks. A demand exits for replacements and/or substitutes which are less expensive and which can be derived from renewable resources.